Ethernet not working, Arris SBG10, No DHCP server was found

All Ethernet connections to my combo modem/router (Arris SBG10) are not working. I have tried multiple computers with multiple cables. The Wi-Fi works fine. Resetting the modem/router didn't help nor did factory resetting the modem/router. All computers I connect with an Ethernet cable have a 169.254 IPv4 address, suggesting the DHCP server is not working. When diagnosing the connection on a Windows 11 device, there is a "No DHCP server was found" error. However, when I login to my modem settings (192.168...), the modem has DHCP enabled. Therefore, I believe that Xfinity is causing the problem. All standard customer service options are abysmal and haven't helped me in the slightest.

How can I get Ethernet to work in my situation?